1. Field of the Invention
The present invention relates to a power storage device including an ionic liquid (also referred to as room temperature molten salt).
Note that the power storage device indicates all elements and devices which have a function of storing power.
2. Description of the Related Art
A lithium secondary battery which is one of power storage devices is used in a variety of applications including mobile phones, electric vehicles (EV), and the like. Characteristics such as high energy density, excellent cycle characteristics, safety under a variety of operating environments, and the like are necessary for the lithium secondary battery.
Many of the widely used lithium secondary batteries include a nonaqueous electrolyte (also referred to as “nonaqueous electrolyte solution”) including a nonaqueous solvent and a lithium salt containing lithium ions. An organic solvent which is often used for the nonaqueous electrolyte is an organic solvent which is liquid at ordinary temperatures and pressures, such as ethylene carbonate which has high dielectric constant and excellent ion conductivity.
However, the organic solvent has volatility and a low flash point. For this reason, in the case where a nonaqueous solvent including the organic solvent is used for an electrolyte solution of a lithium secondary battery, the temperature inside the lithium secondary battery might rise due to a short circuit, overcharge, or the like and the lithium secondary battery might burst or catch fire.
In view of the above, the use of an ionic liquid which has non-flammability and non-volatility as a nonaqueous solvent for a nonaqueous electrolyte of a lithium secondary battery has been proposed.
In the case where the ionic liquid is used for a solvent for the nonaqueous electrolyte of the lithium secondary battery, there is a problem in that a low potential negative electrode material cannot be used because of low reduction resistance of the ionic liquid. Thus, a technique has been disclosed, which enables dissolution and precipitation of lithium which is the low potential negative electrode material without the use of an additive by increasing the reduction resistance of an ionic liquid including a quaternary ammonium salt (see Patent Document 1).